This invention relates to plastic sheet for a laminated glazing and more particularly to controlling adhesion of such sheet to a glazing panel and increasing its resistance to blocking.
Layered, light transmitting safety glazings for window, windshield, sunroof, skylight, intrusion security, showcase, picture frame and like applications are well known. They include one or more rigid transparent panels such as glass combined in a laminate with an impact-dissipating plastic sheet such as plasticized polyvinyl butyral (PVB). It is likewise well established to carefully control the strength of the bond between the plastic sheet and rigid panel at a desired level since if too high the plastic sheet undesirably ruptures on impact and if too low splinters from the rigid panel can separate from the glazing, and, if glass, can injure a person in the surrounding area.
Heretofore, as far as presently known, this bond strength was customarily controlled (as typically disclosed for plasticized PVB in U.S. Pat. No. 3,249,488) by including in the formulation of the sheet chemical adhesion control additives such as alkali and alkaline earth metal salts and bases, metal salts of organic carboxylic and aromatic acids and the like. These anti-adhesion chemical additives which are present in very small quantities in the sheet (generally less than 1/2 weight %) function on a molecular scale and, in the case of polyvinyl butyral with glass where the bond is too strong, are thought to reduce bond strength by occupying sites on the glass and/or plastic sheet which might normally bond to each other. These additives often have an affinity for water and require careful control to avoid absorption of moisture which can produce undesirable haze in the transparent sheet. When present in such small quantities, accurate control of the additives concentration is difficult, particularly on a commercial scale. Since the additives are thought to interact with the glass, variations in the quality of the glass surface affects bond strength which is often not reproducible. Control of the adhesion of plastic sheet to rigid panels in a laminated safety glazing without relying on chemical additives would be desirable.
Furthermore, plasticized PVB sheet notoriously tends to stick to itself (sometimes called "blocking") at ambient temperatures typically encountered during storage before laminating and expensive precautions have been taken to prevent this. For example, the sheet has been transported and stored at low refrigeration temperatures, or interleafed with polyethylene film or dusted between facing layers with sodium bicarbonate. It has been and continues to be very desirable to alleviate this blocking problem associated with plasticized PVB sheet.